Despite improvement in hardware, modern imaging methods including MRI are sensitive to motion of imaging objects such as organs. The sources of motion include physiological motion from heart beating, respiratory motion, as well as voluntary and involuntary movement of patients during imaging procedures. Such motion leads to compromised image quality. In cardiac MRI for example, motion may lead to blurring of heart images, ghosting artifacts of the chest wall and/or other moving organs, or heterogeneous signal distribution across homogeneous tissue, e.g., a myocardium and blood pool. Consequently, the motion may result in non-diagnostic quality images or even false positive or negative findings.
Increased imaging speed improves patient comfort and imaging throughput and reduces the impact of motion on images. State-of-the-art MR scanners are equipped with a strong magnetic gradient system for fast switching of gradient pulses within the limits of peripheral nerve stimulation for increased imaging speed. High density coil arrays are used providing advanced sampling and reconstruction methods for accelerated imaging by under-sampling imaging data. While these features may improve image quality and consistency of results, the degree of motion is still larger than a typical imaging voxel size and the speed of motion is significant compared to image acquisition time in many applications and causes degraded image quality.
Imaging a moving heart has been typically performed by synchronizing data acquisition to an electrocardiogram (ECG) signal. This ensures imaging data may be consistently acquired in the same cardiac phase with same degree of cardiac motion, or acquired in a specific time period (typically mid-diastole or end-systole) comprising minimal cardiac motion. Precise motion control is desirable for many cardiac applications that require high spatial resolution or homogeneous signal. For example, visualization of coronary artery lumen and/or a coronary vessel wall requires sub-millimeter spatial resolution while cardiac motion may be of the order of a centimeter. Tissue characterization relies on subtle difference in MR luminance signal intensity between normal and pathological myocardium and signal intensity may be skewed by motion.
Further, parametric mapping methods (T1, T2, T2* based method) require motion control to derive accurate pixel-wise relaxation parameters. These methods require accurate selection of an image acquisition trigger time to minimize adverse impact on image results from cardiac motion. A system according to invention principles addresses deficiencies of the known imaging methods in imaging in the presence of motion and related problems.